This invention relates to a forging method of producing a hollow part from a cylinder blank.
A conventional method for forging a nut contains the following steps as illustrated in FIGS. 5A-J. A blank 101 is cut so that a diameter D of the blank 101 may be smaller than the width B across the flat portions of the nut by approximately 0.4 mm and also have a large flat cross section. In a first process, the blank 101 are chamfered to produce a cylindrical semi-manufactured product 102. In a second process, a semi-manufactured product 103 having its outer shape preformed to be hexagonal is obtained. In a third process, dents 104a and 104b to be used for forming a female thread are created on a semi-manufactured product 104. In a fourth process, the semi-manufactured product 104 having the dents 104a and 104b is further extruded to form a semi-manufactured product 105 having deeper dents 105a and 105b. Then in a fifth process, piercing is performed to the remaining portion between the dents 105a and 105b, and a final product 106 having a pierced hole 106a is obtained. In the next process, a female thread is formed in the pierced hole 106a. 
However, such a conventional method requires a large number of processes like chamfering and preforming to forge a hollow part, and the processes are complicated.
An object of the present invention is to provide a forging method which enables forging of a hollow part by simplified processes.
In order to attain the above object, the present invention provides a forging method of a hollow part comprising steps of upsetting a cylinder blank having the upsetting ratio of 1.5-2.0 so that the blank has the smaller outer shape and the lower height than those of the part to be formed, forming dents on both sides of the blank in the direction of the height of the part; and piercing the bottom portion of the dents to create a pierced hole in the blank.
It is preferable that the depth of the dents are made different. Also, if the part is selected from the group consisting of a nut, a hub nut and a flange nut, it is preferable to form a female thread after a pierced hole is created. The part could be a bush collar.